The previous fix was wrong because it had two misunderstandings on
freebsd32 calling convention like the following:
- 32-bit id1 implies that it is the upper half of 64-bit id, indeed it
  depends on machine endianness.
- 32-bit ARM calling convension doesn't conform to freebsd32_args,
  indeed it does.

This change fixes the bugs and makes blockUntilWaitable work correctly
on freebsd/{386,arm}.

Fixes #16064.

Change-Id: I820c6d01d59a43ac4f2ab381f757c03b14bca75e
Reviewed-on: https://go-review.googlesource.com/24064Reviewed-by: Ian Lance Taylor <iant@golang.org>